In a conventional optical disk apparatus using a cartridge-type optical disk, a focus servo operation is conducted such that the focal point of a laser beam is brought on the surface of a recording film, in order to achieve accurate recording/reproducing. In the focus servo operation, the objective lens is controlled such that the distance between the objective lens and the optical disk is maintained constant by feeding back a focus error signal. However, it is only within a very small region of approximately ±1 μm that the focus error signal can serve as a linear error signal representing a position of the objective lens relative to the position of the optical disk in the direction of the optical axis of the objective lens. Therefore, it is very difficult to mount a mechanism unit precisely within this region in advance. Therefore, it is necessary to conduct a focus search control for moving the objective lens into a linear region of the focus error signal for conducting the focus servo operation when the optical disk has been inserted. Conventionally, the focus search control is conducted in a method in which the objective lens is gradually brought closer to the focusing point from a position sufficiently far away from the linear region of the focus error signal and a focus servo loop is closed when the focus error signal has entered within a predetermined region.
FIG. 1 shows a block diagram illustrating an overview of a focusing mechanism and a focus control system of a conventional optical disk apparatus. An optical disk 264 is rotated around an axis oriented along arrows X—X, by a spindle motor (not shown). A moving head 262 is movable in directions indicated by arrows Y—Y, i.e., in directions along the radius of an optical disk 264 and, in the moving head 262, an objective lens 270 fixed to a lens holder 268 is mounted through, for example, a pair of plate springs 266. A focusing coil 272 is mounted on the lens holder 268 and moves the objective lens 270 in directions indicated by the arrows X—X by an interaction of a magnetic field generated by a permanent magnet (not shown) fixed inside the moving head 262 and a current flowing in the focusing coil 272. A leading mirror 274 reflects a beam from a fixed head 276 emitted along the direction of the radius of the optical disk 264, in the direction along the axis of the optical disk 264. The beam is gathered by the objective lens 270 onto the surface of the recording film of the optical disk 264. The beam reflected from the optical disk 264 is again reflected by the leading mirror 274 and returned to the fixed head 276.
The fixed head 276 has inside it a laser diode 278, a collimator lens 280, a beam splitter 284, a detector lens 286 and a photo-detector 288. A laser beam emitted from the laser diode 278 is reflected by the beam splitter 284 through the collimator lens 280 and injected into the moving head 262. An outgoing beam from the moving head 262 passes through the beam splitter 284 and is gathered on the photo-detector 288 through the detector lens 286. The photo-detector 288 comprises, for example, a quarter-splitting photodiode and the current being output from the photo-detector 288 is converted to a focus error signal by a focus error signal generating circuit 290.
The focus error signal generally has a shape shown by the dotted line in FIG. 2 and is generated as a signal called S-shaped curve signal when the laser beam from the objective lens 270 has been focused on the surface of the recording film of the optical disk 264. The direction of the axis of abscissas in FIG. 2 indicates the direction in which the objective lens 270 leaves the optical disk 264 relative to the position of the focal point 312. Now, a portion between a point 310 and a point 314 in FIG. 2, i.e., a portion being almost linear between the peaks of the S-shape of the focus error signal can be used as an error signal between the objective lens 270 and the position of the focal point 312. When conducting a focus search, the objective lens 270 is positioned at a position sufficiently away from the focusing position 312 as an initial state. For example, when the objective lens 270 at the balanced position of the plate springs 266 is present in the vicinity of the position of the focal point, the objective lens 270 is brought away from the vicinity of the position of the focal point by ordering a focus current driving circuit 304 to flow a driving current of the focusing coil 272. When the objective lens 270 at the balanced position of the plate springs 266 is present sufficiently away from the position of the focal point, it is enough that a zero (0) is input in the focus current driving circuit 304. At this moment, an initial current value is designated by a ramp circuit 300 and a selection circuit 302 is in a state in which it has selected an output of the ramp circuit 300.
When the focus search has been started, a linear function signal for time is output from the ramp circuit 300 and the current of the focusing coil 272 is controlled by the focus current driving circuit 304 through the selection circuit 302 such that the objective lens 270 approaches to the position of the focal point 312 at a constant velocity. Since a focus actuator having the focusing coil 272 has a frequency characteristic that a displacement in proportion to a DC current is output in response to the DC current, the focus actuator is displaced in a linear function for an input time period as ordered by the ramp circuit 300. Therefore, the objective lens 270 approaches the position of the focal point at a constant velocity and, thereafter, at the vicinity of the position of the focal point, the S-shaped curve portion of the focus error signal as shown in FIG. 2 is output from the focus error signal generating circuit 290.
Then, first, a first comparator 294 monitors the focus error signal and, when this signal exceeds such a predetermined voltage level as indicated by a point 306 in FIG. 2, the first comparator 294 outputs to a second comparator 296 an order to start an operation. Then, the second comparator 296 monitors the focus error signal and, when the signal becomes lower than a predetermined voltage level indicated by a point 308, the second comparator 296 outputs to a third comparator 298 an order to start an operation. Then, the third comparator 298 monitors the focus error signal and, when the signal becomes lower than a voltage level corresponding to the position of the focal point being the point 312, the third comparator 298 outputs an order to the selection circuit 302 to switch. At this moment, an input to the focus current driving circuit 304 is switched from the output of the ramp circuit 300 to the output of a phase compensation circuit 292. In the phase compensation circuit 292, the focus error signal for the vicinity of the focus point, i.e., a position error signal of the objective lens 270 is input and is applied with a phase compensation filtering process and a gain process such as advancing or delaying the phase such that the control system becomes stable when the loop is closed and, therefore, a focus servo control system is formed that works for the objective lens 270 to be always positioned at the position of the focal point. When such a focus search control is conducted, the order from the ramp circuit 300 is an order to move the objective lens 270 at a constant velocity. However, in this method, at the start of the focus search, since the velocity is varied stepwise, the acceleration has an impulse-like shape. This is shown in FIG. 3. The axes of abscissas in FIGS. 3A, 3B and 3C represent time and axes of ordinate represent respectively the displacement of the objective lens in FIG. 3A, the velocity of the objective lens in FIG. 3B and the acceleration of the objective lens in FIG. 3c. At a time zero (0), the focus search starts and an order of a ramp-like shape current shown in FIG. 3A is output from the ramp circuit 300. At this moment, the velocity of the objective lens 270 shown in FIG. 3B is varied stepwise from zero (0) to v0 and, thus, the acceleration working on the objective lens 270 becomes an impulse signal as shown in FIG. 3C. This means that the acceleration generated by the focus actuator contains a high-frequency component.
FIG. 4 shows a frequency characteristic of the focus actuator. In FIG. 4A, the axis of abscissas represents the frequency and the axis of ordinate represents the gain (sensitivity) of the replacement for a unit current input. In FIG. 4B, the axis of abscissas represents the frequency and the axis of ordinate represents the phase angle. The peak at 70 Hz in the frequency characteristic of the focus actuator indicates the main resonance of the actuator and a constant gain for the current, i.e., a displacement in proportion to the current is output at frequencies lower than this. In contrast, when a current input at 70 Hz is applied, the sensitivity becomes 15 dB or higher comparing to that of current inputs at frequencies of 70 Hz or lower and, therefore, the objective lens starts an oscillation.
This situation is shown in FIGS. 5A to 5D. FIGS. 5A, 5B, 5C and 5D show respectively the focus error signal, the focus current, the relative position and the relative velocity of the objective lens. In FIG. 5B, a point 316 indicates the time when the focus search is started, a point 318 indicates the time when the focus is detected, the solid line indicates a track for the case where the focus servo operation is started at the point 318 and the dotted line indicates a track for the case where the focus servo operation is not started at the point 318. The chain lines in FIGS. 5C and 5D indicate respectively the track of the targeted position and the track of the targeted velocity. It is understood that, even when the focus search current varies in a ramp-like shape as described above, the resonance of the focus actuator considerably influences on a practical position trajectory and the velocity track and, therefore, those tracks are considerably away from the targeted tracks. This considerably influences adversely on a stable focus search. Since the region in which the focus error signal can be used as an error signal is limited, a normal feedback control can not operate if a great overshoot occurs in the response at the time of starting of the focus servo operation due to an influence of the initial velocity exceeding the designed value. Considering that a further narrowing of the region in which the focus error signal can be used is advanced when a short-wavelength light source is employed as a factor in the shift to larger capacities of optical disk apparatuses in the future, this can be said to be a fatal problem.